An active matrix substrate used for a liquid crystal display apparatus or the like is provided with a switching element such as a thin film transistor (hereinafter “TFT”) for each pixel. As such a switching element, TFT having an amorphous silicon film as an active layer (hereinafter, “amorphous silicon TFT”) and a TFT having a polycrystalline silicon film as an active layer (hereinafter, “polycrystalline silicon TFT”) are widely used in the related art.
In recent years, use of an oxide semiconductor instead of amorphous silicon or polycrystalline silicon as a material for an active layer of a TFT has been proposed. Such TFTs are referred to as “oxide semiconductor TFTs”. Oxide semiconductors have higher mobility than amorphous silicon. Therefore, oxide semiconductor TFTs are able to operate at a higher speed than amorphous silicon TFTs. In addition, in oxide semiconductor TFTs, rising of Id-Vg characteristic is steep and an off leak current is small since the oxide semiconductors have the higher mobility.
Meanwhile, techniques are known in which a drive circuit such as a gate driver or a source driver is monolithically (integrally) provided on a substrate (for example, refer to PTL 1). These drive circuits (monolithic drivers) are usually formed using TFTs. Recently, a technique of manufacturing a monolithic driver on a substrate using an oxide semiconductor TFT has been used which allows a reduction in costs to be realized by narrowing a pixel-frame region and simplifying a mounting process.
Generally, TFTs forming a drive circuit (hereinafter referred to as “circuit TFTs”) are manufactured in a step of manufacturing a TFT arranged as a switching element for each pixel (hereinafter referred to as “pixel TFTs”) at a same time. Therefore, the circuit TFTs and the pixel TFTs are formed using a same oxide semiconductor film and often have a same or a similar structure. Usually, an enhancement-type TFT having a positive threshold voltage Vth is used as the pixel TFT and the circuit TFT.
Even display units of active matrix type liquid crystal display apparatuses using an active matrix substrate may not be immediately cleared when a user turns off power supply and whitish and blurry images may remain thereon. The reason for this is that when the power supply of the apparatus is turned off, a discharge path of charges held in a pixel capacitor is interrupted and a residual charge is accumulated in a pixel region. In addition, when the power supply of the apparatus is turned on in a state where the residual charge is accumulated in the pixel region, deterioration of a display quality, such as generation of flickering based on the residual charge, occurs.
In a liquid crystal panel monolithically provided with a gate driver (hereinafter referred to as a “gate driver monolithic panel”), in addition to charges in a display region and charges of a gate bus line, it is also necessary to discharge charges on a floating node in a monolithic gate driver (the charges on the two floating nodes indicated by reference symbols netA and netE described below). In the monolithic gate driver using an a-Si TFT, since the off-leak current of the a-Si TFT is relatively large, the charges on the floating nodes in the monolithic gate driver (which may be hereinafter referred to as “floating charges”) are able to be discharged in approximately three milliseconds. However, in a monolithic gate driver using an oxide semiconductor TFT having a small off-leak current, it is difficult to promptly discharge floating charges and there is a possibility that it will not be possible to sufficiently suppress charge unevenness caused by the floating charges.
On the other hand, the present inventors found that using a depression-type oxide semiconductor TFT having a negative threshold voltage Vth makes it possible to reduce an amount of charge which is accumulated in the display region and in drive circuits such as a monolithic gate driver.
FIG. 12 is a diagram illustrating current and voltage characteristics of an enhancement-type oxide semiconductor TFT and a depression-type oxide semiconductor TFT. The horizontal axis represents a gate voltage Vgs based on a source and the vertical axis represents a drain current Ids. In the enhancement-type TFT, a threshold voltage Vth is positive (Vth>0(V)), and the leakage current Ids is suppressed to be small when the gate voltage Vgs is 0 V. In this example, the leakage current Ids is approximately 4 pA. On the other hand, in the depression-type TFT, the threshold voltage Vth is negative (Vth<0(V)), and the leakage current. Ids when the gate voltage Vgs is 0 V is greater than that of the enhancement-type TFT. In this example, the leakage current Ids is approximately 260 nA. As above, in the depression-type oxide semiconductor TFT, it is possible to increase the leakage current when the gate voltage Vgs is 0 V (hereinafter, the leakage current which is generated when the gate voltage Vgs is 0 (V) is represented as “Ids(Vgs=0)”. Therefore, it is considered that, when a depression-type oxide semiconductor TFT is used, it is possible to rapidly discharge floating charges in a display region and in a drive circuit via the oxide semiconductor TFT, and to suppress gate bus line defects, charge unevenness, and the like caused by the floating charges.